r/o bind mounts: filesystem helpers for custom 'struct file's

Why do we need r/o bind mounts?

This feature allows a read-only view into a read-write filesystem.  In the
process of doing that, it also provides infrastructure for keeping track of
the number of writers to any given mount.

This has a number of uses.  It allows chroots to have parts of filesystems
writable.  It will be useful for containers in the future because users may
have root inside a container, but should not be allowed to write to
somefilesystems.  This also replaces patches that vserver has had out of the
tree for several years.

It allows security enhancement by making sure that parts of your filesystem
read-only (such as when you don't trust your FTP server), when you don't want
to have entire new filesystems mounted, or when you want atime selectively
updated.  I've been using the following script to test that the feature is
working as desired.  It takes a directory and makes a regular bind and a r/o
bind mount of it.  It then performs some normal filesystem operations on the
three directories, including ones that are expected to fail, like creating a
file on the r/o mount.

This patch:

Some filesystems forego the vfs and may_open() and create their own 'struct
file's.

This patch creates a couple of helper functions which can be used by these
filesystems, and will provide a unified place which the r/o bind mount code
may patch.

Also, rename an existing, static-scope init_file() to a less generic name.

Signed-off-by: Dave Hansen <haveblue@us.ibm.com>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 9 insertions, 9 deletions

diff --git a/net/socket.c b/net/socket.c
index 3cd96fe8191d..540013ea8620 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -364,26 +364,26 @@ static int sock_alloc_fd(struct file **filep)
 
 static int sock_attach_fd(struct socket *sock, struct file *file)
 {
+        struct dentry *dentry;
         struct qstr name = { .name = "" };
 
-        file->f_path.dentry = d_alloc(sock_mnt->mnt_sb->s_root, &name);
-        if (unlikely(!file->f_path.dentry))
+        dentry = d_alloc(sock_mnt->mnt_sb->s_root, &name);
+        if (unlikely(!dentry))
                 return -ENOMEM;
 
-        file->f_path.dentry->d_op = &sockfs_dentry_operations;
+        dentry->d_op = &sockfs_dentry_operations;
         /*
          * We dont want to push this dentry into global dentry hash table.
          * We pretend dentry is already hashed, by unsetting DCACHE_UNHASHED
          * This permits a working /proc/$pid/fd/XXX on sockets
          */
-        file->f_path.dentry->d_flags &= ~DCACHE_UNHASHED;
-        d_instantiate(file->f_path.dentry, SOCK_INODE(sock));
-        file->f_path.mnt = mntget(sock_mnt);
-        file->f_mapping = file->f_path.dentry->d_inode->i_mapping;
+        dentry->d_flags &= ~DCACHE_UNHASHED;
+        d_instantiate(dentry, SOCK_INODE(sock));
 
         sock->file = file;
-        file->f_op = SOCK_INODE(sock)->i_fop = &socket_file_ops;
-        file->f_mode = FMODE_READ | FMODE_WRITE;
+        init_file(file, sock_mnt, dentry, FMODE_READ | FMODE_WRITE,
+                  &socket_file_ops);
+        SOCK_INODE(sock)->i_fop = &socket_file_ops;
         file->f_flags = O_RDWR;
         file->f_pos = 0;
         file->private_data = sock;